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ALLOWANCE FOR LOAN LOSSES - Allowance by Impairment Analysis Method (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment, Recorded Investment	$ 25,142	$ 19,187
Collectively Evaluated for Impairment, Recorded Investment	7,079,167	6,957,517
Total loans	7,104,309	6,976,704
Individually Evaluated for Impairment, Allowance for loan losses	400	235
Collectively Evaluated for Impairment, Allowance for loan losses	80,838	71,747
Allowance for loan losses	81,238	71,982	$ 75,312	$ 74,742
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment, Recorded Investment	4,315
Collectively Evaluated for Impairment, Recorded Investment	554,028
Total loans	558,343
Individually Evaluated for Impairment, Allowance for loan losses	279
Collectively Evaluated for Impairment, Allowance for loan losses	8,706
Allowance for loan losses	8,985	10,048
Paycheck Protection Program
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment, Recorded Investment	0
Collectively Evaluated for Impairment, Recorded Investment	312,356
Total loans	312,356	0
Individually Evaluated for Impairment, Allowance for loan losses	0
Collectively Evaluated for Impairment, Allowance for loan losses	159
Allowance for loan losses	159	0
Commercial tax-exempt
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment, Recorded Investment	0
Collectively Evaluated for Impairment, Recorded Investment	442,159
Total loans	442,159	447,927
Individually Evaluated for Impairment, Allowance for loan losses	0
Collectively Evaluated for Impairment, Allowance for loan losses	2,550
Allowance for loan losses	2,550	6,016
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment, Recorded Investment	5,262	733
Collectively Evaluated for Impairment, Recorded Investment	2,752,113	2,550,541
Total loans	2,757,375	2,551,274
Individually Evaluated for Impairment, Allowance for loan losses	50	0
Collectively Evaluated for Impairment, Allowance for loan losses	51,111	40,765
Allowance for loan losses	51,161	40,765	41,934	46,820
Construction and land
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment, Recorded Investment	0	0
Collectively Evaluated for Impairment, Recorded Investment	159,204	225,983
Total loans	159,204	225,983
Individually Evaluated for Impairment, Allowance for loan losses	0	0
Collectively Evaluated for Impairment, Allowance for loan losses	4,041	5,119
Allowance for loan losses	4,041	5,119	6,022	4,949
Residential
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment, Recorded Investment	14,942	15,900
Collectively Evaluated for Impairment, Recorded Investment	2,662,522	2,823,255
Total loans	2,677,464	2,839,155
Individually Evaluated for Impairment, Allowance for loan losses	54	67
Collectively Evaluated for Impairment, Allowance for loan losses	12,810	8,790
Allowance for loan losses	12,864	8,857	10,026	9,773
Home equity
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment, Recorded Investment	623	1,830
Collectively Evaluated for Impairment, Recorded Investment	76,741	81,827
Total loans	77,364	83,657
Individually Evaluated for Impairment, Allowance for loan losses	17	22
Collectively Evaluated for Impairment, Allowance for loan losses	276	756
Allowance for loan losses	293	778	1,284	835
Consumer and other
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment, Recorded Investment	0	0
Collectively Evaluated for Impairment, Recorded Investment	120,044	134,674
Total loans	120,044	134,674
Individually Evaluated for Impairment, Allowance for loan losses	0	0
Collectively Evaluated for Impairment, Allowance for loan losses	1,185	399
Allowance for loan losses	$ 1,185	399	134	630
Commercial and Industrial Portfolio Segment, Including Tax Exempt
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment, Recorded Investment		724
Collectively Evaluated for Impairment, Recorded Investment		1,141,237
Total loans		1,141,961
Individually Evaluated for Impairment, Allowance for loan losses		146
Collectively Evaluated for Impairment, Allowance for loan losses		15,918
Allowance for loan losses		$ 16,064	$ 15,912	$ 11,735